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A reconnaissance geological study has been carried out in the Vartdal area of the basal gneiss region of S.W. Norway. The country rock is amphibolite facies gneiss of granodioritic composition affected by 4 folding phases, in part of Precambrian age. The gneiss contains pods of marble, amphibolite, metadolerite, anorthosite, diorite and eclogite. The chemical variation within the Sunnmore Metadolerite Suite is explained by igneous processes whilst the development of garnet- and biotite-rich corona assemblages is ascribed to autometamorphism rather than prograde regional metamorphism which caused amphibolitisation. The Sunnmore Eclogite Suite comprising 131 samples has been the subject of reconnaissance field and mineralogical studies and detailed petrographical and geochemical investigations. The field relationships provide clear evidence for a tectonic emplacement. 32 mineral types and 35 mineral reactions are recorded and a petrographic classification is proposed based on incompatibilities between certain minerals. 44 individual minerals have been analysed. Two distinct eclogite facies are recognised. 64 new whole-rock analyses for 13 major elements are presented. The suite shows a wide chemical variation but within certain orthopyroxene eclogite pods two types of trend can be identified, one best explained by high pressure igneous processes and the other by subsequent metamorphic differentiation. Estimates by chemical geothermometry and geobarometry suggest 800°C for the major eclogite re-equilibration at pressures up to 35kb. A review of the literature on experimental petrology indicates the feasibility of generating olivine-free eclogite from olivine-bearing mantle. Some possible tectonic models of eclogite genesis are discussed and an orogenic mixing process within a subduction zone emplacing mantle-derived eclogites into downwarped continental crust is favoured. Details of the chemical analytical techniques and of four computer programs specially developed for this project are described in appendices.